Login failer to my DB using ASP.NET 2.0???

Using Visual Studio 2005 Pro with the accompanying install of SQL Server Express.  I'm running a Visual C# GUI that reads from SQL Server a DB that I also need to be read from a ASP.NET 2.0 app that I'm running in IIS 5.1 my GUI has a user of 'TUDOR\Windows' and works perfectly but when I try to make a connection in my ASP.NET app I get a debug error (see below).  In the design view I can set the sqldatasource, bind it to a datagridview and test my query but if I debug it just doesn't work and the same when I visit the site, just gives the debug error of:

Cannot open database "ATSDB" requested by the login. The login failed.
Login failed for user 'TUDOR\ASPNET'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: Cannot open database "ATSDB" requested by the login. The login failed.
Login failed for user 'TUDOR\ASPNET'.

I really know nothing about SQL Server, dbo_owners, security etc what do I need to do to get this going   I really need this to work as I'm running out of time.  The GUI is done and static parts of the site are done too just need to hook up the dynamic bits by Wed morning.  Any help would be much appreciated :)